Conventional mechanical seals are employed in a wide variety of mechanical apparatuses to provide a pressure-tight and a fluid-tight seal between a rotating shaft and a stationary housing. The seal is usually positioned about the rotating shaft, which is mounted in and protrudes from the stationary housing. The seal is typically bolted to the housing at the shaft exit, thus preventing loss of pressurized process fluid from the housing. Conventional mechanical seals include face type mechanical seals, which include a pair of annular sealing rings that are concentrically disposed about the shaft, and axially spaced from each other. The sealing rings each have seal faces that are biased into physical contact with each other. Usually, one seal ring remains stationary, while the other ring contacts the shaft and rotates therewith. The relatively rotating, contacting seal faces isolate and seal a pressurized liquid, i.e., the process fluid, along the rotating shaft. The mechanical seal prevents leakage of the pressurized process fluid to the external environment by biasing the seal ring sealing faces into physical contact with each other.
To cool the seals and to aid in preventing any passage of process fluid across the seal faces, a second pressurized liquid, i.e., a barrier fluid, is often introduced to the seals on the side of the seal faces opposite that in contact with the process fluid. Springs normally bias the seal faces together. In balanced seal arrangements, the pressurized fluids are also applied to piston areas defined on the sides of the seal members opposite the seal faces to aid in closing the seal faces. This relationship minimizes heat generation from the frictional contact of the seal faces while maintaining a closing force on the seal faces sufficiently high to ensure proper sealing. It is also desirable to minimize the contact area of the seal faces so as to minimize heat generation as the seal faces rotate relative to each other. Additionally, when a barrier fluid is employed, a double seal arrangement is utilized in which the process fluid is confined to one end of the seal and the barrier fluid to the center of the seal with relatively rotating seal faces on either side of the barrier fluid.
In one type of double balanced seal in the prior art, both fluids have access to the rear of the seal members opposite the seal faces, and the desired balance ratio of the piston area to the seal face contact area is achieved by providing O-rings slidable in their O-ring grooves behind the respective seal faces of the seal members. Thus, the O-rings slide in the grooves to permit application of fluid pressure from the fluid having the highest pressure to the appropriate piston areas on the sides of the seal members opposite the seal faces. Springs may be located within the seal on either side of the seal faces and may be exposed to either or both of the process and barrier fluids.
Prior double-balanced mechanical seal assemblies have significant drawbacks. First, the piston areas in prior double-balanced mechanical seal assemblies are dependent upon the size and configuration of the O-rings. As the inner and outer diameters of the O-rings define the balance pressure points for the respective fluids, the radial contact dimension of the seal faces must be sufficiently large to account for the thickness of the O-rings. This limits the design of the seal faces for which minimum contact area is desired to reduce heat generation.
An additional drawback of double-balanced mechanical seal assemblies of the prior art is that the double-balanced seal does not operate efficiently under reverse pressure conditions. Under reverse pressure conditions, the O-rings slide in their grooves to achieve sealing. Furthermore, the process fluid, which may be dirty and include contaminants, causes dirt and other particles to get caught in the sliding O-ring interface, which causes wear and O-ring hang-up over time, thereby negatively impacting seal performance.